If you mounted the M2s against the wall, they'll benefit from significant reinforcement of bass frequencies (adjacent surfaces), so you might try 80 Hz. I think 100 Hz is too high if they're mounted to a wall.

Peter, the crossover setting made by the receiver indicates that the speaker in your setting(i.e., on the wall)had enough bass capability(helped by the room reinforcement)to justify a 60Hz crossover. The fact that it was capable of 60Hz doesn't necessarily mean that this is ideal and it should be left there. With your fine sub, it should be given more of the load and less given to the M2s. As Alan suggested, 80Hz should be suitable.